Planet X was a place-holder name used by astronomers in their search for a planet beyond the orbit of Neptune. They found Pluto (but later
lost it demoted it to a "dwarf planet"). It is also the term favoured in several apocalyptic predictions and conspiracies to refer to a hypothetical planet that will kill us all.
 In astronomy
In the period when Pluto was still considered a planet, the name was used again for any hypothetical trans-Neptunian "tenth planet," with the "X" doubling as the Roman numeral for "10". The discovery of Eris and other objects caused people to realise that there was no good definition of "planet", so they made one... and had to demote Pluto (and to promote the asteroid Ceres, but most people don't seem to give a damn about it).
The existence of additional bodies like Pluto were initially predicted by looking at the orbits of the other planets - a model with 8 planets didn't seem to stack up against the evidence. If any other significant planets existed, but had yet to be directly spotted, their gravitational effect would be very visible as perturbations in the orbits of the other known planets and would account for the discrepancies then thought to exist between the planets' predicted orbits and their observed orbits. As it happened, the errors in observation (and the errors in guessing the known planets' masses) completely accounted for the discrepancies, and no astronomer today believes there are any undiscovered massive bodies perturbing the observed orbits.
In the 1990s and early 2000s, Nancy Lieder claimed that a tenth planet - Planet X - would pass by Earth in 2003, causing cataclysmic events, possibly topped with an alien invasion. After the date passed, the Planet X story became incorporated in the 2012 apocalypse claims. It is supposed to be the same as Zecharia Sitchin's "twelfth planet" Nibiru and the names are often used interchangeably or joined with a slash ("Planet X/Nibiru"), but Sitchin's original prediction was for the return of Nibiru in 2085. There is very little evidence that any planet like this exists.
Planet X has been accused of being Wormwood, the star from the Book of Revelation prophesized to smash into us. As if that won't destroy us. Also: A planet is not a star.
 See also
- Bad Astronomy - The Planet X Saga, Phil Plait's debunking page at the old Bad Astronomy site, dealing with Nancy Lieder's and Mark Hazlewood's original claims about Planet X coming in 2003.
- Planet X at 2012hoax.org
- All Planet X posts at the Exposing Pseudoastronomy blog
- Planet X, episode #159 of Astronomy Cast (covers both the search for a trans-Neptunian planet and the conspiracy theories)
- Things You Need To Do Before Announcing You have Found Niburu (or Planet X or Tyche), some advice to wannabe doomsday planet discoverers on YouTube (and elsewhere)
- ↑ Note that by "errors", scientists don't mean "mistakes." They mean the imprecision with which existing equipment could make measurements.